Chapter 7 Chapter 7 Conclusions and Future Scopes
3.3 Short-term Fatigue and Bending Analysis
The morphology of the fractured surface of composite is studied under LEO 1430 VP Scanning Electron Microscope supplied by LEO Electron Microscopy Inc. The fractured surface of the specimen is prepared by gold coating (as the composite is non-conductive) and examined under high voltage (10kV) with 500 times magnification. Figure 3.4 illustrates the micrograph of the fractured surface of 15% cement filled polypropylene composite. The result shows adequate dispersion of cement particles in the polypropylene matrix. The results thus validate the certain bonding between the matrix and filler material. Further, it is anticipated that the bonding is improved and enhanced due to the presence of grafted polypropylene.
Figure 3.4 SEM image of the fractured surface of 15% cement filled composite with 500 magnification
Chapter 3 Investigation and Evaluation of Mechanical Properties…
loops evaluated during the experiments. A typical example is shown at each stress level under respective loading conditions. The results indicate that the deformation behavior of the composite specimen depends on the loading conditions and the cycle selected.
0 0.2 0.4 0.6 0.8 1
0 0.25 0.5 0.75 1 1.25
Load (kN)
Extension (mm)
0%
5%
10%
15%
Figure 3.5 Load vs. extension of the composite materials
0 5 10 15 20 25
0 4000 8000 12000 16000 20000 24000 28000
Stress (MPa)
Strain (x 10-6)
0%
5%
10%
15%
Figure 3.6 Stress–strain curve indicates hysteresis of the composite materials
The tension-compression effect of the specimen is observed across the thickness while maintaining the same frequency of the cyclic load is applied. This is considered to be the reason that the fatigue life of the composite specimen is highly dependent on the loading TH-1430_09610309
conditions. Therefore, not only the stress level but also the strain should be taken into consideration to explicate the fatigue behavior of specimens under various loading conditions (Sugimoto and SasakiI-II, 2007).
Further, strain energy i.e. mechanical work done per unit volume of the composite specimen is calculated and illustrated in the Figure 3.7(a). The strain energy reflects the deformation behavior of a specimen during loading from zero to the peak value. The fatigue behavior of polymeric composite material causes significant dissipation of energy under loading- unloading condition. Energy dissipation ratio, denoted by Hc, can be defined as the ratio of energy loss per cycle, i.e. measured from the area enclosed by the loading-unloading curve to the strain energy per cycle, i.e. area enclosed by the loading curve as shown in the Figure 3.7(b). The energy dissipation ratio is used as an index of fatigue performance of the composite material (Sugimoto and sasaki, 2008).
0 100 200 300 400
0 20 40 60 80 100
0 5 10 15
Strain energy (kJ/m3 ) Dissipated energy (kJ/m3 )
Cement content (wt%)
0.09 0.11 0.13 0.15 0.17
0 100 200 300 400
0 5 10 15
Energy dissipation ratio(Hc) Dissipated and strain energy (kJ/m3 )
Energy dissipated (kJ/m3) Strain enengy (kJ/m3) Energy dissipation ratio
Filler (wt%)
(a)
Chapter 3 Investigation and Evaluation of Mechanical Properties…
0 0.05 0.1 0.15 0.2
0 5 10 15
Energy dissipation ratio (Hc)
Cement content (wt%)
(b)
Figure 3.7 Variation of (a) dissipated strain energy and (b) energy dissipation ratio with the filler loading (wt%)
Clorius et al., (2000) demonstrated that, the stiffness decreases during the fatigue life of the materials and gives a fatigue life dependent elastic modulus, E(t). The time dependant elastic modulus in unloading cycle, denoted by EU(t), is not same as the time dependent elastic modulus, EL(t), in loading condition. The ratio of EU(t) to EL(t) is denoted as the stiffness ratio. The stiffness of the material is observed to be fatigue life dependant and is governed by the time dependant elastic modulus. This stiffness ratio is obtained in loading-unloading sequences is determined at any given time of the fatigue life of the materials and can be expressed as:
Stiffness ratio (χ)
U
L
E t
E t
(3.1)
The stiffness ratio (χ) depends on the filler loading %, as such the stiffness ratio increases with increase of filler materials. The total loading-unloading cycle time has been assumed as a total fatigue life of a sample because the experiment has been done in order to evaluate short term fatigue performance of the composite materials. The total experimental time has segregated into four relative fatigue life of each sample and it is observed that fatigue life significantly varies with the stiffness ratio as shown in the Figure 3.8.
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0.8 1 1.2 1.4 1.6 1.8
0.25 0.5 0.75 1
Stiffness ratio (Ф)
Relative life
0%
5%
10%
15%
Figure 3.8 Variation of stiffness ratio with relative life
3.3.2 Bending Deformation Analysis
To understand the shear deformation under loading condition, the 3-point bending tests of composite materials are conducted in universal testing machine (INSTRON- 8801) followed by the ASTM standard D790. The load-deflection graph of the test results are represented in the Figure 3.9.
0 5 10 15 20 25 30
0 2 4 6 8 10 12 14 16 18 20 22
Load (N)
Deflection (mm)
0%
5%
10%
15%
Figure 3.9 Load-deflection behavior of composite materials
Chapter 3 Investigation and Evaluation of Mechanical Properties…
From the test results, the interlaminar shear stress, which is at a maximum at the neutral axis, is calculated with the aid of the standard relationship (Christiansen et. al., 1972). The tensile and the shear stresses are calculated for the composite (where it is maximum) and is given by the expression:
3P L 2BD D
(3.2)
3P
4BD(3.3) where τ is the interlaminar shear stress at the neutral axis, P is the applied load, B the width and D the thickness of the specimen. The load is measured at the first major maximum in the load deflection curve as shown in Figure 3.9. The results obtained for the complete range of composite samples using the load deflection results are illustrated in the Figure 3.10.
Figure 3.10 Variation of tensile stress and shear stress vs. deflection of the composite materials
In the Figure 3.11, the demarcation of failure region over tensile and shear has shown. The straight line represents the boundary between tensile and shear failures according to inequality which differentiates between shear and tensile failure of the composite materials (Daniels et al., 1971).
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0 5 10 15 20 25 30 35
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55
Tensile strength (MPa)
Shear strength (MPa)
Shear
Tensile
Figure 3.11 Tensile strength vs. shear strength at the failure region